1. Field of the Invention
Embodiments disclosed herein relate generally to cutting structures used to drill wells in the earth. More specifically, embodiments disclosed herein relate generally to materials used for expandable downhole reaming tools.
2. Background Art
In the drilling of oil and gas wells, typically concentric casing strings are installed and cemented in the borehole as drilling progresses to increasing depths. Each new casing string is supported within the previously installed casing string, thereby limiting the annular area available for the cementing operation. Further, as successively smaller diameter casing strings are suspended, the flow area for the production of oil and gas is reduced. Therefore, to increase the annular space for the cementing operation, and to increase the production flow area, it is often desirable to enlarge the borehole below the terminal end of the previously cased borehole. By enlarging the borehole, a larger annular area is provided for subsequently installing and cementing a larger casing string than would have been possible otherwise. Accordingly, by enlarging the borehole below the previously cased borehole, the bottom of the formation can be reached with comparatively larger diameter casing, thereby providing more flow area for the production of oil and gas.
Various methods have been devised for passing a drilling assembly through a cased borehole, or in conjunction with expandable casing to enlarging the borehole. One such method involves the use of an underreamer, which has basically two operative states—a closed or collapsed state, where the diameter of the tool is sufficiently small to allow the tool to pass through the existing cased borehole, and an open or partly expanded state, where one or more arms with cutters on the ends thereof extend from the body of the tool. In this latter position, the underreamer enlarges the borehole diameter as the tool is rotated and lowered in the borehole.
A “drilling type” underreamer is one that is typically used in conjunction with a conventional “pilot” drill bit positioned below (i.e. downstream of) the underreamer. Typically, the pilot bit drills the borehole to a reduced gauge, while the underreamer, positioned behind the pilot bit, simultaneously enlarges the pilot borehole to full gauge. Formerly, underreamers of this type had hinged arms with roller cone cutters attached thereto. Typical former underreamers included swing out cutter arms that pivoted at an end opposite the cutting end of the cutting arms, with the cutter arms actuated by mechanical or hydraulic forces acting on the arms to extend or retract them. Representative examples of these types of underreamers are found in U.S. Pat. Nos. 3,224,507; 3,425,500 and 4,055,226, all incorporated by reference herein.
Examples of hydraulically expandable, concentric reaming rools are also described in U.S. Pat. Nos. 4,431,065 and 6,732,817. In the '065 patent, a tubular body includes a recess having a cutting arm received therein. The cutting art is moved between a retraced position approximately aligned with the axis of the tubular body and a deployed or activated position extending laterally outwardly of the body by a hydraulic plunger that actuates the cutting arms from a fully retracted to a fully deployed position.
Another device that has been developed is the near-bit reamer. Near-bit reamers may be run into a wellbore with typical steerable BHAs, and the near-bit reamers are generally activated downhole by, for example, hydraulic pressure. When activated, a pressure differential is created between an internal diameter of the reamer and a wellbore annulus. The higher pressure inside the reamer activates pistons that radially displace a reamer cutting structure. The reamer cutting structure is typically symmetrical about a wellbore axis, including, for example, expandable pads that comprise cutting elements. The cutting elements are moved into contact with formations already drilled by the drill bit, and the near-bit reamer expands the diameter of the wellbore by a preselected amount defined by a drill diameter of the expanded reamer outing structure.
While these tools are effective in enlarging/stabilizing a borehole, they are generally considered to be not ideal tools for use when drilling with turbines, for example. Turbines are frequently used in deep wells for longer drilling, as the use of turbines allows for high RPMs* (and greater ROPs) with lower energy and WOB inputs. As the motors or turbines powering the bit improve (higher sustained RPM), and as the drilling conditions become more demanding, the durability of bits and other downhole tools such as reamers also needs to improve. Accordingly, there exists a continuing need for improvements in downhole tools, such as reamers.